1. Field of the Invention
This invention concerns an image forming method that is excellent in terms of the reliability of high-quality images and can be used favorably in image formation by the electrophotography method, etc.
2. Description of the Related Art
The electrophotography method and other methods of making image information visible through electrostatic images are presently used widely in a variety of fields. Generally with the electrophotography method, an electrostatic image is developed on a photoconductor via a charging process, an exposure process, etc., and the electrostatic image is made visible via a transfer process, a fixing process, etc.
A fixing device for the fixing process is generally equipped with a heating roll, and by making a recording medium, which holds a toner image formed from unfixed toner, pass between the heating roll and a pressure roll or other member, the image is fixed onto the recording medium.
In a generally employed method of heating the heating roll, a heat-generating heater, such as a halogen lamp heater, is incorporated and the surface of the heating roll is heated by the radiant heat from the heater. However, in cases where the heat-generating heater is used as the heat source, the heat transfer efficiency is poor since heat is transferred to the heating roll via air with this structure, and the time for heating to a temperature necessary for fixing the toner (the so-called warming-up time) is thus long. Also, since both ends of the heating roll are opened, air from the exterior tends to become mixed in readily and the temperatures at the ends thus tended to be lower in comparison to the central part of the heating roll. Though there are methods in which the resistance at both ends of the heat generating part of the heat-generating heater is increased to prevent the temperature drop at both ends of the heating roll, such methods tended to make the heat generating part complex in structure and lead to such problems as increased costs, increased power consumption, etc.
As a heating roll, with which the warming-up time can be shortened and uniformity of temperature of the heating roll can be maintained, Japanese Patent Laid-Open No. 189381/1984 proposes a heating roll, with which a resistive heat generator, made of a substance that generates heat upon passage of electricity, is formed into a roller. Also, Japanese Patent Laid-open No. 213480,1992 proposes a method of passing electricity uniformly through the resistive heat generator, Japanese Patent Laid-Open No. 332331/1994 proposes a method of further shortening the warming-up time by defining the temperature coefficient of resistance of the heat-generating element, and Japanese Patent Laid-Open No. 127818/1997 proposes a method of preventing leakage of electricity during passage of electricity through the resistive heat generator. With these methods, since the surface or the vicinity of the surface f the heating roll directly heated by the resistive heat generator, the surface of the heating roll can be heated to a predetermined temperature rapidly and yet uniformly after the start of the passage of electricity through the resistive heat generator.
Generally with a fixing device that employs such a heating roll, a circular power-receiving ring member, which is electrically connected to the resistive heat generator and rotates along with the resistive heat generator, and a conducting part, which is arranged to provide electricity to the resistive heat generator via a feeding member that is in contact with the receiving ring member, are employed to pass electricity through the resistive heat generator.
Generally in the case where a heating roll, such as described above, is employed to fix a toner image, formed from unfixed toner held on a recording medium, the warming-up time is shorter in comparison to an abovementioned heating roll that employs a halogen lamp heater or other type of heat-generating heater. However, the surface temperature of this heating roll is lowered by the contact with the recording medium, and this makes it difficult to obtain the gloss and coloration demanded of the fixed image and leads to such problems as causing non-uniformity of gloss and coloration of toner image on the same recording medium. These problems become especially significant in the case where fixing is to be performed using color toners since the gloss and coloration required of color toners are generally higher than those required of a monochromatic toner.
Also, in the case where fixing is performed under high temperature and high humidity, since the moisture contained in the recording medium evaporates due to contact with the heating roll, the non-uniformity of gloss and non-uniformity of coloration are worsened. Though, for example, a method, wherein the same type of heating device is provided at the opposing pressure roll as well to restrain the temperature drop of the surface of the heating roll, may be considered for solving the above problem, this method is not adequate in effect and also has problems of increased amount of electricity conducted, etc.
Meanwhile, there is also a method wherein a conductive member is disposed at the surface or the vicinity of the surface of the heating roll and a magnetic field is made to act on the conductive member so that heat is generated at the surface or the vicinity of the surface of the heating roll by the resulting eddy current (Japanese Patent Laid-Open No. 301415/1998). Since this arrangement is one in which the surface or the vicinity of the surface of the heating roll is made to generate heat directly, the warming-up time can be shortened and the uniformity of the temperature of the heating roll can be maintained as in the case of the heating roll that uses a resistive heat generator. However, the same problems of non-uniformity of gloss and non-uniformity of coloration as the heating roll that uses a resistive heat generator still remain.
Furthermore, with regard to the amount of power required by these arrangements in which the surface or the vicinity of the surface of the heating roll is made to generate heat directly, improved efficiency is realized only for the input energy amount required for fixing. Conventionally, a binder resin for toner that is used in image forming by the electrophotography method is an amorphous resin made up of a non-crystalline resin, and in the case where image preservation under a realistic condition of approximately 50xc2x0 C. is considered, the fixing temperature required for fixing is at least approximately 130xc2x0 C. or more. There is thus a limit to fundamental measures for achieving low consumption power with regard to the energy required for fixing.
This problem likewise applies in the case of a method wherein a toner image formed on a photoconductor is subject to primary transfer onto an intermediate transfer medium of low non-uniformity of surface properties and electrical properties, with this primary transfer being electrostatically carried out in a successively overlaying manner in the case of multiple colors, and then the abovementioned multiple color toner image formed on the intermediate transfer medium is subject to secondary transfer onto a recording medium and thereafter fixed by a fixing device. That is, even in this case, a heating roll is generally used in the fixing device, and the same problems as the above will occur if an arrangement is employed wherein the surface or the vicinity of the surface of the heating roll is made to generate heat directly.
Furthermore, with regard to so-called simultaneous transfer and fixing methods, wherein an intermediate transfer medium, onto which a toner image formed from unfixed toner has been transferred, and a recording medium are superimposedly inserted and heat-fixed, with the toner image contacting the recording medium, between a transfer and fixing device having a pair of heating rolls and/or pressure rolls, etc., methods have been disclosed wherein a conductive member is disposed at the surface or the vicinity of the surface of the intermediate transfer medium and a magnetic field is made to act on the conductive member to make the surface or the vicinity of the surface of the intermediate transfer medium generate heat in advance by means of the resulting eddy current (Japanese Patent Laid-Open No. 352804/1999, Japanese Patent Laid-Open No. 242108/2000, Japanese Patent Laid-Open No. 275982/2000, etc.). However, since these methods also employ an arrangement wherein the surface or the vicinity of the surface of an intermediate transfer medium that contacts the toner image is made to generate heat, the surface temperature of the intermediate transfer medium is lowered by contact with the recording medium. The same problems of non-uniformity of gloss and non-uniformity of coloration thus exist as in the heating roll cases and there is also a limit in terms of measures for achieving low power consumption.
As has been described above, in all cases of an arrangement wherein in the process of fixing by heating a toner image formed from unfixed toner, the surface or the vicinity of the surface of a heating roll, intermediate transfer medium, or other heating member that contacts the toner image is made to generate heat, a temperature drop occurs when the recording medium contacts the heating member and this leads to non-uniformity of gloss and non-uniformity of coloration. Furthermore, even when any of these various heat transfer methods that are good in thermal efficiency is employed, adequate heat for melting the toner to the required degree has to be applied and thus there is a limit to fundamental measures for achieving low consumption power with regard to energy.
In view of the above circumstances, this invention therefore provides an image forming method with which the warming-up time is short and which enables high-quality images to be formed at high speed and even upon continuous fixing. This invention also provides an image forming method with which the lowering of image quality, such as non-uniformity of gloss, and non-uniformity of coloration, can be restrained especially even under high-temperature high-humidity conditions. This invention furthermore provides an image forming method that uses a toner that is wide in fixing range and is particularly excellent in low-temperature fixing property.
The image forming method according to the present invention includes a process of heating a heating member that is in contact with a toner image to thereby melt the toner and fix the toner image on the record medium. The heating member is a member with which the surface or the vicinity of the surface in contact with the toner image generates heat, and the toner contains a colorant and a binder resin, which resin containing a crystalline resin, as the main component, with a number average molecular weight of approximately 1500 or more.
Generally in comparison to a non-crystalline resin, a crystalline resin has a melting point and thus exhibits a large lowering of viscosity at a specific temperature. Since the temperature difference between the point at which the resin molecules begin thermal activity and the range in which fixing is possible can thus be made small, the resin can be made one that is excellent in low-temperature fixing property. This is an advantage that is not provided by non-crystalline resins with which the resin molecules begin thermal activity at the glass transition point and decreases in viscosity gradually.
When a crystalline resin with such characteristics is used as a binder resin for a toner, since a degree of melting that is adequate for fixing can be attained as long as a temperature greater than or equal to the melting point can be secured, the toner will be wide in fixing range and especially excellent in low-temperature fixing property.
With the present invention, a toner, having a crystalline resin with the abovementioned characteristics as the main component of the binder resin, is applied to an image forming method that uses a heating member with which the surface or the vicinity thereof generates heat in the fixing process. The benefits of shortening of warming-up time and saving of energy, which are the merits of the fixing process, are thus provided while restraining the lowering of image quality, such as non-uniformity of gloss, and non-uniformity of coloration, even when an abovementioned recording medium contacts the heating member and causes a temperature drop. Also, a low-temperature fixing property can be achieved to enable further savings in energy.
The present invention can thus prevent the lowering of image quality that is due to temperature drop of the heating member surface, resulting from contact of the heating member with a recording medium or evaporation of the moisture in the recording medium under high-temperature and high-humidity conditions, as well as that due to an inadequate amount of heat resulting from higher speeds, etc.